This invention relates to a clock recovery method and system for use in a ring-shaped synchronization network to recover a sequence of clock pulses.
A conventional ring-shaped synchronization network of the type described comprises a plurality of transmission devices which are connected through transmission lines or paths to one another in a ring shape and which are bidirectionally communicable with one another. In such a synchronization network, the transmission devices are classifiable into a master transmission device for delivering a sequence of clock pulses to the synchronization network and a plurality of slave transmission devices each of which is operable in response to the clock pulse sequence. The clock pulse sequence may be called a master clock pulse sequence and is circulated in a selected one of clockwise and counterclockwise directions that will be referred to as a selected direction. Each of the slave transmission devices is operated in accordance with an internal clock pulse sequence which is synchronized with the master clock pulse sequence.
Specifically, each of the slave transmission devices is supplied through the transmission line with the master clock pulse sequence from an upstream transmission device, such as the master or the slave transmission device, to produce the internal clock pulse sequence with reference to the master clock pulse sequence extracted by each slave transmission device. On the other hand, each slave transmission device transmits the master clock pulse sequence through the transmission line to a downstream transmission device which may be the slave or the master transmission device. Thus, network synchronization is established in the network by transmitting the master clock pulse sequence in the selected direction.
With this structure, it often happens that a fault takes place on the transmission line between two adjacent ones of the transmission devices and that the master clock pulse sequence can not be transferred to the downstream transmission device or devices in the selected direction. As a result, the network synchronization is frequently disturbed or stalled.
Herein, it is assumed that such a fault takes place on the transmission line for transmitting the master clock pulse sequence and that a specific one of the slave transmission devices detects the fault. In this event, the selected direction for the master clock pulse sequence may be switched to a reversed direction in the specific slave transmission device.
Heretofore, the direction of the master clock pulse sequence has been manually and successively switched from the selected direction to the reversed one until the specific transmission device. In addition, manual switching has also been carried out on recovery of the fault to change the direction from the reversed direction to the selected direction.